This answer quotes ChatGPT
Your query statement is incorrect. The correct SQL query statement is.
SELECT student.sname, student.class FROM student WHERE student.sno NOT IN ( SELECT elective.sno FROM elective WHERE elective.cno = 'C06' );
This query uses a subquery to find the number of a student taking course C06 on elective relational table Elective table, and then finds a student name and class that does not appear in this subquery result in Student relational table Student.